The present invention relates to an information-transmission system for performing communications or broadcast via non-geosynchronous artificial satellites.
There are currently many systems using a geosynchronous artificial satellite in the communications or broadcast field. A method using one geosynchronous artificial satellite has an advantage in that it is possible to continuously serve a specific district on the earth with one artificial satellite. In this method, the artificial satellite can be seen in a fixed direction from the district, and therefore, seen only if a receiver on the earth orients an antenna in the fixed direction toward the artificial satellite. In the communications field also, mobile communication apparatuses have come into practical use, and when a communication is carried out by a mobile communication apparatus, the mobile communication apparatus can receive the communication service by adjusting the direction of its antenna toward the geosynchronous artificial satellite.
On the other hand, a communication service which uses non-geosynchronous artificial satellites, has been now offered. In this communication service, when a mobile communication apparatus initiates its communication, a non-geosynchronous artificial satellite, which is located at the most suitable position for the communication, is selected. Further, an optimally selected satellite may change during the communication due to the movement of the mobile communication apparatus or that of the artificial satellite used for the communication. Accordingly, in the communication service which uses non-geosynchronous artificial satellites, the artificial satellite being used is changed over to another artificial satellite, and in this switching, a handover process is performed in order to maintain the quality of the communication.
In the communication or broadcast service, low orbit-, elliptical orbit-, or figure-eight orbit-artificial satellites are used, and since they move in their own predetermined orbits, they do not stop at positions relative to points on the earth. If a continuous communication or broadcast service is offered, a plurality of artificial satellites are launched, and they are spaced and moved in the same orbit in predetermined time intervals. Then, while the satellite whose position is optimal for the communication is selected, the communication or broadcast service will be continuous. In the above communication or broadcast service system, the surface of the earth would be served by a plurality of artificial satellites, with the optimal artificial satellite for the communication being selected from among a plurality of candidates.
Moreover, another non-geosynchronous artificial satellite communication or broadcast system, which is used for a specific region on the surface of earth, is possible, in which a plurality of artificial satellites is arranged on a plurality of orbits so that at least one satellite always exists at a position suitable for the communication or broadcast of the specific region, and by switching the satellites in order, the communication or broadcast is continued. In the system, which uses a geosynchronous artificial satellite, since the satellite stops at a position in midair over the equator, an elevation angle toward the satellite is smaller as the latitude of the place, at which signals are received from the satellite, becomes larger. Accordingly, in the regions of high latitude on the surface of earth, for example, Japan, if a geosynchronous artificial satellite system is used, the elevation angle toward the satellite is small. Thus, a problem occurs in which, if a receiving apparatus is a mobile device, the radio wave sent from the satellite is shielded by buildings, mountains, etc., during movement of the receiving apparatus. This has been a severe defect in a system which uses a geosynchronous artificial satellite. Therefore, weight has recently been attached to the above-described system which uses non-geosynchronous system. Although a plurality of satellites is necessary in a system which uses non-geosynchronous artificial satellites, the region which can receive the communication or broadcast service from an artificial satellite communication system, can be greatly extended, and communication or broadcast systems, which use the above-described elliptical orbit- or figure-eight orbit-artificial satellites, have been devised. If the communication or broadcast system, which covers all over the land of Japan, is realized with the above non-geosynchronous artificial satellites, it is calculated that a large elevation angle of about 70 deg. can be always ensured by appropriately arranging three satellites on the orbits so that they round on their orbits with the time interval of 8 hours, and are used in order. However, such a communication system has not been put to practical use, and subjects for practical use of this system must be reviewed.
Meanwhile, communication systems, which switch the use of artificial satellites in order, are disclosed, for example, in Japanese Application Laid-Open Hei 11-034996 and Japanese Application Laid-Open Hei 1-272235.
In a communication or broadcast system, which uses artificial satellites arranged on the non-geosynchronous orbits, such as elliptical orbit- or figure-eight orbit-artificial satellites, if the system is designed so that a single satellite is used for a region which receives the service from the system, the artificial satellites, to be used, are switched in order. The inventors have realized that troubles in the communication or broadcast tend to occur in that switching of the satellites, and these troubles must be prevented in order to put the above system to practical use. Various kinds of troubles can be assumed. For example, there is the hazard in that a large quantity of radio wave packets may be invalidated. It has become clear that the switching of the satellites may cause a deterioration in reliability of the system, to the user.